This invention relates to a pump positioned at a drill bit. It relates, more particularly, to a pump which aim it is to clean the drill bit and at the same time limit the fluid pressure that is acting against the abutment surface and that may cause leakage of drilling fluid into the rock during the drilling of wells, for example such as those drilled for the exploration and production of oil or gas.
During the drilling of wells in the underground, ripped rock, usually called cuttings, is washed away and up from the drilling surface by means of drilling fluid which is pumped down inside the drill string and flushed out through nozzles in the drill bit in order then to flow up the annulus formed between the drill string and the established borehole. In addition to the hole cleaning which is described above, through its adapted specific weight, the drilling fluid is to hold back inflow from possible pressurized layers in the underground. However, the specific weight of the drilling fluid together with the reactive circulation pressure acting against the drilling surface must not exceed such a value that the drilling fluid leaks into, or even breaks open, fractures in the rock that is being drilled. If such leakages should occur, the drilling fluid will disappear and lead to circulation failure with subsequent impaired hole cleaning and risk of inflow from pressurized layers.
It is not unusual for geological layers and pressures in the underground to be such that it is necessary to drill with specific weights and circulation rates that give a total fluid pressure relatively close to a value that could be sufficient to break up the rock. The risk of breaking open fractures is usually greatest at the drilling surface where new rock is being exposed and where the hydrostatic pressure column together with the reactive circulation pressure is at its highest. At the same time, more unpredictable pressure components from turbulence and high velocities are also acting in this surface. An optimum solution to reduce the risk of forcing open fractures has such properties that it limits the total fluid pressure against the drilling surface to a value equal to the fluid pressure that is acting in the established borehole. In this way, the fluid pressure against new rock or sediment will not exceed a level approximately equal to the one that has been found bearable so far in the drilling process and that, by means of known techniques, may be read continuously through transponders and telemetry.
In U.S. Pat. No. 5,775,443 and the corresponding WO application 2008/055349 an ejector pump to be built into a drill bit for the purpose of improving the cleaning of a drill bit is described. The purpose is achieved by directing a side flow from the internal bore of the drill string to ejector pumps placed externally on the drill bit whereas the main flow is carried to the ordinary nozzles of the drill bit. Thus, by passing drilling fluid at high pressure through the nozzles of the drill bit in the ordinary way, no restriction is established in the pressure acting against the drilling surface, and thereby the solution has no effect in relation to limiting the fluid pressure against the drilling surface. Further, the solution has the considerable drawback of being integrated in the drill bit and not being connected to the drill bit through a standard coupling. The range of drill bits is thereby restricted, which is critical in relation to the value of being able to select a drill bit on the basis of acquired local experience and change the type of drill bit according to changes in the nature of the rock.